The synthetic resin formed by the polymerization of propylene as the sole monomer is called polypropylene. While "polypropylene" has been used from time to time in the art to include a copolymer of propylene and a minor amount of another monomer, such as ethylene, the term is not so used herein.
The well-known crystalline polypropylene of commerce is a normally solid, predominantly isotactic, semi-crystalline, thermoplastic polymer mixture formed by the polymerization of propylene by Ziegler-Natta catalysis. In such catalysis the catalyst is formed by an inorganic compound of a metal of Groups I-III of the Perodic Table, (for example, an aluminum alkyl), and a compound of a transition metal of Groups IV-VIII of the Periodic Table, (for example, a titanium halide). A typical crystallinity is about 60% as measured by X-ray diffraction. As used herein, semi-crystalline means a crystallinity of at least about 5-10% as measured by X-ray diffraction. Also, the typical weight average molecular weight (Mw) of the normally solid polypropylene of commerce is 100,000-4,000,000, while the typical number average molecular weight (Mn) thereof is 40,000-100,000. Moreover, the melting point of the normally solid polypropylene of commerce is about 162.degree. C.
Although the polypropylene of commerce has many desirable and beneficial properties, it is deficient in melt strength. When molten, it exhibits no strain hardening (an increase in resistance to stretching during elongation of the molten material). Thus it has a variety of melt processing shortcomings, including the onset of edge weave during high speed extrusion coating of paper or other substrates, sheet sag and local thinning in melt thermoforming, and flow instabilities in coextrusion of laminate structures. As a result, its use has been limited in such potential applications as, for example, extrusion coating, blow molding, profile extrusion, and thermoforming.
On the other hand, low density polyethylene made by a free radical process has desirable melt theology for applications that require melt strength or strain hardening properties. Such low density polyethylene is believed to have these properties because the polymer molecules are non-linear. The molecules are chains of ethylene units that have branches of ethylene units. This non-linear structure occurs because of typical free radical inter- and intra-molecular transfer followed by further subsequent polymerization.
Low molecular weight, amorphous (predominantly atactic) polypropylene with a branched molecular structure is known in the prior art. See Fontana, Kidder and Herold, Ind. & Eng. Chem., 44 (7), 1688-1695 (1952), and the U.S. Pat. No. 2,525,787, to Fontana et al. It is disclosed as having been made by Friedel-Crafts catalysis. However, the molecular weight (weight average) of this polypropylene is at most about 20,000, the polymer is described as having normal (at 20.degree. C.) viscosity ranging from that of a light lubricating oil to that of a heavy oil or even resins of plastic or semi-solid nature, and its utility is reported to be as a blending-agent and viscosity index improver for lubricating oils.
The crystalline polypropylene of commerce, however, is linear. That is, the polymer molecules are chains of propylene units without branches of propylene units. The reason is that in Ziegler-Natta catalysis secondary free radical reactions such as occur in the free radical polymerization of ethylene are highly improbable, if not non-existent.
Some effort has been made in the art to overcome the melt strength deficiency of the crystalline polypropylene of commerce.
Thus, as reflected in the U.S. Pat. No. 4,365,044, to Liu, and cited references thereof, blending of linear polypropylene with a low density polyethylene that does have desirable melt strength or strain hardening properties, alone or with other polymeric substances, has been tried with some success. However, the blend approach involving different polymeric substances is not preferred.
Another approach to improve the melt properties of linear polypropylene is disclosed in the U.S. Pat. No. 3,349,018, to Potts. According to this patent, linear polypropylene is degraded by subjecting it in air to ionizing radiation at a total dose from about 0.01 to about 3 megareps (equivalent to about 0.012 to about 3.6 megarads), but less than a dose at which gelation is caused. This patent discloses that radiation degraded linear polypropylene can be extruded and drawn at much higher linear speeds without the occurrence of draw resonance or surging. However, as can be determined from the patent, particularly Example VI, the neck-in of the in-air irradiated linear polypropylene is actually greater than the neck-in of the non-irradiated linear polypropylene.
There are a number of references that disclose the ionizing radiation treatment of linear crystalline polypropylene. These references, however, describe the resulting polymer either as degraded, as a result of chain scisson, or as crosslinked, as a result of polymer chain fragments linking together linear polymer chains. There seems to be very little true recognition, if any, in these references of the possibility of an intermediate condition in which the product of the treatment is a polypropylene having "dangling" or free-end long branches.
For example, one such reference is Marans and Zapas, J. Appl. Pol. Sci., 11, 705-718 (1967). This reference reports experiments in which samples of a powdered, crystalline, linear propylene in sealed glass tubes are subjected at pressures less than 0.3 millimeters of mercury to electron radiation at various doses of radiation, and then heated to 175.degree. C. to melt the irradiated polypropylene. The authors of this reference characterize the irradiated polypropylene of the samples as cross-linked. However, in connection with the instant invention, duplicative experiments and more advanced measuring techniques have indicated that Marans and Zapas had in fact obtained polypropylene with free-end long branches. On the other hand, the reference contains no disclosures of utility of the irradiated and heat treated samples.
Geymer, Die Makromolekulare Chemie, 99, 152-159, (1969 No. 2230), discloses experiments in which a crystalline, linear, polypropylene was subjected in a vacuum to gamma ray radiation from cobalt 60, and afterwards exposed to methyl mercaptan (to minimize oxidative degradation on exposure of the irradiated polymer to air), and then exposed to air. While the reference states that the simultaneous fracture and cross-linking result in branched molecules, no utility of the resulting propylene polymer material is disclosed. Moreover, while the reference does not disclose the dose rate of the gamma radiation, the usual dose rate from the usual cobalt 60 source is of the magnitude of about 1 Mrad. per hour. In view of work done in connection with the instant invention, the extent of branching without cross-linking in the Geymer experiments, therefore, is believed to have been insignificant.